The present invention relates to an emission type computed tomography (CT) apparatus.
Recently, an emission type CT apparatus has been developed in which .gamma. rays emitted from radioisotope given to an organism are detected and a tomogram is reconstructed on the basis of an RI distribution within the organism. From a safety viewpoint, the emission type CT is advantageous since no X rays are radiated into the organism, but has in a poor field ratio. The reason for the disadvantage is that a .gamma. ray detector of the emission type CT can detect only the .gamma. rays projected orthogonally to a detecting surface, that is to say, a field of the detector is confined within the same area as the detecting surface. When the detector rotates in a plane containing a cross section of the organism, the detecting surface is generally in parallel with an axis of the organism, including a tangential line of a locus circle of the rotation. A tomogram of a cross section of the organism obtained is always within a circle area with a diameter equal to the width of the cross sectional area of the detector. For this reason, in order to obtain tomogram of cross sectional areas of a head and an abdomen of a human body by the same detector, the width of the cross sectional area of the detector must be made large conforming to the size of the abdomen. This measure, however, is unadvisable because the detector with a large field is expensive and a drive mechanism for driving it is made large in size, then the CT apparatus is made large in size with poor handling of the apparatus.
Also for obtaining a longitudinal tomogram along a body axis of a human body, the following problem arises. For taking a longitudinal tomogram of the entire abdomen longer than a length of the longitudinal width of the detector, the human body or the detector must be moved along the body axis by several scannings. This is a time consuming work and needs an additional moving mechanism.